1. Field of the Invention
The present invention relates to annealing of a semiconductor thin film by irradiation with laser light.
2. Description of the Related Art
Extensive studied have been made of laser annealing techniques in which an amorphous or crystalline semiconductor thin film formed on a substrate is illuminated with laser light to crystallize or improve the crystallinity of the semiconductor thin film.
In particular, by using a laser light having a line-shaped cross section (linear laser light, hereinafter), annealing can be performed uniformly with high productivity.
A description will be made of linear laser light. Usually, laser light as output from a laser light source has a spot-like cross-section of a several centimeter square. This laser light is expanded and made uniform by a homogenizer or a beam expander, and then converged by a cylindrical lens. Thus, linear laser light is obtained which has a size of several millimeters by several tens of centimeters on an irradiation surface.
By inserting a slit between the cylindrical lens and the irradiation surface, the energy density profile of the linear laser light in its line-width direction on the irradiation surface is rendered rectangular, to enable more uniform annealing.
The laser light source is a pulsed laser having large output power, such as an excimer laser. After a crystalline silicon film is annealed with laser light of a low energy density, annealing is again performed with laser light of a high energy density.
This procedure can not only provide larger crystal grain diameters but also reduce the degree of roughening of a film as compared to the annealing only with laser light of a high energy density.
That is, in crystallizing or improving the crystallinity of a thin film of a semiconductor such as silicon by using laser light, it is important in terms of reduction in the degree of film roughening to irradiate the thin film first with laser light of a low energy density and then with laser light of a higher energy density.
However, such multiple-stage laser irradiation has a problem that the annealing process takes loner time than in single-stage laser irradiation.
One method for solving this problem is to make the energy density profile of linear laser light in its line-width direction trapezoidal or Gaussian.
A trapezoidal energy density profile can be obtained by controlling the distance between the final-stage cylindrical lens (focusing lens) and the irradiation surface in an optical system for forming laser light having a rectangular energy density profile which system has a slit.
A Gaussian energy density profile can be obtained by removing a slit or widening the slit in the line-width direction of linear laser light in an optical system for forming laser light having a trapezoidal energy density profile.
If the irradiation surface is irradiated with linear laser light having such a trapezoidal energy density profile while being scanned with it, laser light of a low energy density is applied to the irradiation surface first, and thereafter laser light of an increasingly high energy density is applied.
Therefore, a coating is not irradiated suddenly with laser light of a high energy density unlike the case of using laser light having a rectangular energy density profile. Thus, the coating can be crystallized satisfactorily by single scanning on the entire irradiation surface. Almost the same results are obtained also in the case of using laser light having a Gaussian energy density profile.
When laser annealing is performed by using laser light having a trapezoidal or Gaussian energy density profile, the inventors of the present invention discovered that an annealed semiconductor film is occasionally roughened in a strip-like manner; that is, a plurality of stripes are formed in the longitudinal direction of the linear laser light.
One reason of this stripe-like film roughening is that depending on the energy density profile of laser light, different locations on the irradiation surface may not be irradiated with laser light of the same energy density. This will be explained below with reference to FIGS. 6A-6D, with respect to a case of crystallizing an amorphous silicon film by laser annealing.
FIGS. 6A-6D show a scanning process with linear laser light having a trapezoidal energy density profile. More specifically, FIGS. 6A-6D show a case where linear laser light having a trapezoidal energy density profile which is emitted from a pulsed laser light source such as an excimer laser is moved in the scanning direction by a pitch D for each shot of irradiation. In FIGS. 6A-6D, characters xcex1xe2x80x2 and xcex2xe2x80x2 indicate positions on the irradiation surface and E1 and E2 represent energy density levels (E1 less than E2).
In a first shot, as shown in FIG. 6A, a laser light having a trapezoidal energy density profile is applied to a portion near position xcex1xe2x80x2.
Next, in a second shot, the laser light that is moved by the pitch D is applied as shown in FIG. 6B. The energy density of laser light at position xcex1xe2x80x2 is E1. As a result, an amorphous silicon film in the vicinity of position xcex1xe2x80x2 is crystallized well.
It is assumed that an amorphous silicon film is crystallized well at the energy density E1, and that the crystallinity of a silicon film obtained by irradiation at the energy density E2 is improved properly.
In the second shot, on the other hand, almost no laser light is applied to a portion in the vicinity of position xcex2xe2x80x2. (Although actually there is a portion in the vicinity of position xcex2xe2x80x2 which is irradiated at a low energy density, no change in film quality occurs there because the energy density is much lower than E1.)
In a third shot, laser light that is further moved by the pitch D is applied as shown in FIG. 6C. The energy density of laser light at position xcex1xe2x80x2 is E2 . As a result, the crystallinity of a crystalline silicon film in the vicinity of position xcex1xe2x80x2 is improved.
However, a portion in the vicinity of position xcex2xe2x80x2 is suddenly irradiated at an energy density higher than E1. As a result, an amorphous silicon film in the vicinity of position xcex2xe2x80x2 is crystallized but roughened to a large extent.
In a fourth shot, as shown in FIG. 6D, The energy density of laser light at position xcex1xe2x80x2 is again E2.
In laser crystallization of a silicon film, the film quality obtained at a certain position of the silicon film is greatly influenced by laser light that is applied there first and has a sufficiently high energy density to change the film quality.
In other words, second application onward to a certain position of laser light whose energy density is approximately the same as or lower than in first application to the same position is not important to the resulting film quality at that position.
Therefore, second application to position xcex1xe2x80x2 of laser light having the energy density E2 does not much affect the film quality at that position.
On the other hand, at the time of the fourth shot, the film is already roughened at position xcex2xe2x80x2. Although the crystallinity is improved by the irradiation at the high energy density E2, the resulting film quality at this position becomes different than at other positions.
Thus, the intrasurface uniformity in film quality is impaired, resulting in a problem that a plurality of semiconductor devices formed on the same substrate by using the above silicon film have different characteristics.
In addition, there is a problem of output power variation of a laser light source. A pulsed laser light source (oscillator) produces laser beams of a given energy density at a predetermined frequency (i.e., pulse interval). However, there may occur an event that the output power of the laser light source suddenly decreases. Usually, reduction in output power occurs at a rate of several shots per tens to hundreds of shots. It is rare that output power reduction occurs consecutively.
FIGS. 7A-7D show a scanning process with linear laser light in which the output power of a laser light source varies. As in the case of FIGS. 6A-6D, the energy density profile of laser light in its line-width direction is trapezoidal in FIGS. 7A-7D.
FIGS. 7A-7D show a case where linear laser light having a trapezoidal energy density profile is moved in the scanning direction by a pitch D for each shot of irradiation. In FIGS. 7A-7D, character xxe2x80x2 indicates a specific position on the irradiation surface.
In a first shot, laser light is applied as shown in FIG. 7A. It is assumed that in the first shot an output power reduction of the laser light source has caused a reduction xcex94E in the energy density of laser light. At this time, almost no laser light is applied to position xxe2x80x2.
It is assumed that also in a second shot a reduction xcex94Exe2x80x2 in the energy density of laser light as shown in FIG. 7B. At this time, laser light having a very low energy density is applied to position xxe2x80x2, and hence almost no crystallization occurs there.
It is assumed that in a third shot laser light having the normal energy density is applied as shown in FIG. 7C. At this time, position xxe2x80x2 is suddenly irradiated with laser light having a sufficiently high energy density to effect crystallization, thus roughening the film.
Even if fourth-shot laser irradiation is performed as shown in FIG. 7D, the film quality of the semiconductor thin film is not improved.
That is, although the crystallinity is improved, the film quality is rendered non-uniform.
The above problems occur equally in the case of laser light having a Gaussian energy density profile.
An object of the present invention is to improve the intrasurface uniformity of an annealed semiconductor thin film in crystallizing or improving the crystallinity of a semiconductor thin film by irradiating, while scanning, it with linear laser light.
Another object of the invention is to anneal a semiconductor thin film uniformly over a substrate surface even if there occurs a reduction in the output power of a laser light source.
According to one aspect of the invention, there is provided a laser irradiation apparatus comprising means for producing linear pulse laser light having a step-like beam profile in a line-width direction, the beam profile including step sections each having a given irradiation energy density and a length Ln in the line-width direction; and means for irradiating an irradiation object with the laser light while scanning the irradiation object with the laser light at a pitch D in the line-width direction, wherein the length Ln and the pitch D satisfies Lnxe2x89xa7D.
According to another aspect of the invention, there is provided a laser irradiation method comprising the steps of producing linear pulse laser light having a step-like beam profile in a line-width direction, the beam profile including step sections each having a given irradiation energy density and a length Ln in the line-width direction; and irradiating an irradiation object with the laser light while scanning the irradiation object with the laser light at a pitch D in the line-width direction, wherein the length Ln and the pitch D satisfies Lnxe2x89xa7D.
In the above laser irradiation apparatus and method, it is preferred that a relationship Lnxe2x89xa73D be satisfied. This is because irradiating a specific location of the irradiation object two or more times with pulse laser light having the same irradiation energy density can prevent a variation in irradiation effect due to a variation in the irradiation energy density of laser light.
To prevent a variation in the irradiation effect of laser light, it is even preferred that a relationship Lnxe2x89xa75D be satisfied.
An example of the step-like beam profile is a profile shown in FIG. 1 which has two steps of irradiation energy densities. In this case, the step sections having the lengths Ln are two sections having lengths L1 and L2. In general, the number of step sections is a natural number that is larger than 1.
In the example of FIG. 1, the beam profile includes a first section having an energy density E1 and a length (in the line-width direction of linear laser light) L1=L1 (n=1) and a second section having an energy density E2 and a length L2=L2 (n=2).
The energy density E1 may be set at a value suitable for rendering the irradiation object into a first state while the energy density E2 may be set at a value suitable for rendering the irradiation object into a second state.
For example, an amorphous silicon film can be crystallized efficiently by irradiation with laser light by setting the energy density E1 at a value suitable for crystallizing the amorphous silicon film and setting the energy density E2 at a value suitable for improving the crystallinity of a crystallized silicon film.